1. Field of the Invention
This invention generally relates to systems and methods for providing illumination of a specimen for inspection. Certain embodiments relate to a system configured to illuminate a diffuser with a predetermined pattern of coherent laser light and to image light exiting the diffuser onto an illumination pupil of the system thereby reproducing the predetermined pattern at the illumination pupil.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
One obvious way to improve the detection of relatively small defects is to increase the resolution of an optical inspection system. One way to increase the resolution of an optical inspection system is to decrease the wavelength at which the system can operate. As the wavelength of inspection systems decrease, incoherent light sources are incapable of producing light with sufficient brightness. Using light sources that have sufficient brightness is important to successful inspection since using a light source with relatively low brightness can reduce the sensitivity of the inspection system. In particular, when using a relatively low brightness light source, the signal-to-noise ratio of output signals generated by the inspection system may be too low for accurate defect detection. To mitigate the effects of a low brightness light source on the output signals of an inspection system, the throughput of inspection may be reduced to allow enough light to be collected. Obviously, reduced throughput for inspection is highly undesirable.
Accordingly, for inspection systems that are designed to operate at smaller wavelengths, a more suitable light source is a laser light source that can generate relatively bright light at relatively small wavelengths. However, laser light sources generate coherent light. Such light is disadvantageous for inspection since coherent light can produce speckle in images of a specimen generated by the system. Since speckle is a source of noise in the images, the signal-to-noise ratio of output signals generated by the inspection system will be reduced by speckle. Therefore, many illumination systems have been developed for inspection applications that reduce the speckle of light from laser light sources.
Coherent light is also disadvantageous in imaging-based inspection systems since the coherent light can produce ringing in images generated by the inspection systems. In particular, coherent light will produce sharp transitions in the images instead of smooth transitions. These sharp transitions can produce artifacts in inspection images that will increase the difficulty of detecting defects in the inspection images. Reducing the coherence of the light such that the specimen is illuminated with incoherent or partially coherent light will decrease the aberrations in the images of the specimen. Therefore, for inspection applications, many illumination systems have been developed that reduce the coherence of the light generated by a coherent light source before the light impinges on the specimen.
One illumination system that can be used with a coherent light source includes a diffuser that is illuminated by a coherent laser beam. In one such illumination system, the surface of the diffuser is imaged onto the entrance of a homogenizing rod. The Fourier transform plane of the diffuser surface is imaged into the system pupil, and the exit of the homogenizing rod is imaged into the system field. In such a system, the scatter distribution from the diffuser determines the light distribution in the system pupil. Speckle modulation in the image is minimized by rotating the diffuser during the integration time of the imaging sensor.
Another illumination system that can be used with a coherent light source includes a coherent laser beam that is scanned around the system pupil using galvo mirrors, a spinning polygon mirror, or a holographic scanner. The divergence angle of the spot in the pupil establishes the distribution of light in the field. The resulting image is a sum of all of the coherent images formed during the integration time of the imaging sensor.
Yet another illumination system that can be used with a coherent light source includes a stationary or rotating diffuser that is illuminated by a coherent laser beam. The diffused beam is scanned around the system pupil by galvo mirrors positioned at the Fourier transform plane of the diffuser. The diffuser surface is imaged into the entrance of a homogenizing rod. The resulting image is a sum of all of the coherent images formed during the integration time of the imaging system.
Each of the illumination systems described above, however, has at least one of the following disadvantages. For example, the distribution of light in the pupil is difficult to control. Specific distributions of light can be generated in the pupil using apertures, but light is lost due to the blocking of the light by the apertures. In another example, the distribution of light in the field is not uniform. Better uniformity can be achieved by sacrificing light and illuminating an area that is larger than the field of view of the imaging sensor. In an additional example, power densities are high on surfaces near pupil planes. High power densities can hasten surface contamination, damage optical coatings, and damage bulk glass materials. In yet another example, multiple mechanisms are required (e.g., a spinning diffuser and scanning mirrors). Mechanisms increase the complexity of the system, increase the costs of parts, and are common failure points.
Accordingly, it would be advantageous to develop systems and methods for illuminating a specimen for inspection that use a coherent light source but illuminate the specimen with light that is not coherent and that provide ease of control over the distribution of light in the pupil of the systems, control over the distribution of light in the pupil without a substantial loss of light, a substantially uniform distribution of light in the field without losing a substantial amount of light, relatively low power densities on surfaces of optical components, a relatively simple configuration having relatively few mechanisms, or some combination thereof.